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This test was originally added to fix https://crbug.com/578554 which was
that the autoscroll animation when holding down the mouse button over
the scrollbar track would overshoot the cursor. The test added there was
testing this in a round about way: it would click twice in the same
location on the track, assuming that the animation would start but not
finish before the second click. In this way, it was checking that the
calculation logic was using the final position of the thumb.

However, this makes an assumption about frame timing and animations. It
likely became flaky in the threaded suite with the introduction of the
compositor scrollbar controller, which would try to update the animation
in this case.

This CL replaces the test with a more direct check for the original bug
by holding the mouse button down over the track and waiting for the
scrollTop value to settle.

Note, the test fails consistently on Mac with compositing enabled. This
is due to the Mac-specific expectation for page scrolling. This specific
value is implemented only on the main thread so when compositor threaded
scrollbars are used, we get the same value as on other platforms. See
https://crbug.com/1133538

Bug: 915926,901906,854910
Change-Id: I4e5774a0ee8452ac31b6d199b5d2c68d67feca7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438677Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812368}